The present invention relates to strings of lights used for decorating during holiday times.
Typical light strings consist of plural lamps electrically wired together in series or in parallel. There is an electrical plug at one end of the string adapted for being inserted into a wall socket or other source of electricity, and a receptacle at the other end adapted to receive a plug from a second light string and to deliver electricity to a second light string. Most lights strings operate on alternating, household current. More recently, some light strings have been made to operate on direct current produced by a rectifier placed in the plug or in the receptacle, as disclosed, for example, by the present applicant in U.S. Pat. No. 5,777,868, U.S. Pat. No. 5,994,845 and U.S. Pat. No. 6,869,313. These light strings use less electricity and operate at lower temperatures, so they are less of a fire hazard than more conventional light strings that rely on alternating current. The advantage of the rectifier located in the plug is that only two wires are needed to bring electricity to the lamps. However, this type of direct current light string will deliver direct current to the receptacle at the other end, so the next light string must also operate on direct current.
The advantage of placing the rectifier in the receptacle is that the receptacle can deliver alternating current to the next light string, while delivering direct current to the lights in the first string. Unfortunately this arrangement requires an additional wire.
Light strings are made in the hundreds of millions of sets each year. Containing manufacturing costs is a significant objective of manufacturers of light strings, particularly when the costs of materials increase. Keeping costs low or reducing costs by reducing the amount of material and labor can be significant in the aggregate even when the incremental material or labor costs for a single string of lights is very small.
Additionally, as another way of improving light strings while keeping costs low, instead of incandescent lamps, LEDs are being used more and more for lights in light strings. LEDs as diodes, and unlike incandescent lamps, pass current in only one direction are therefore limited to direct current applications.
While LEDs operating on DC use less current and are cooler to the touch, they are not shunted as many conventional incandescent bulbs are. Shunts are to provide current flow to keep remaining lamps in the circuit lighted in the event of a filament burn out or mechanical failure. In incandescent lights, the filaments can act as fuses. However LEDs do not have shunts because most LEDs short out as hot junctions tend to melt the anode/cathode metals and they pool together forming a short circuit. Shorting of LEDs thus presents a problem that is not addressed in the prior art.
Thus there remains a need for producing LED light strings that are safe, use little electricity, are inexpensive to manufacture and still produce the desired decorative effect, while keeping manufacturing costs low.